I'm wondering if these were cancelled orders through Amazon rather than returns.

Yep, you are correct. This was a canceled Amazon order. The buyer made a $300 order and canceled it the same day. Per Amazon's return policy (and ours) we have to honor that. Since that rarely happens on our site, we used to always absorb returns like this. However, Amazon returns/cancellations are nearly 10 times more common. So I finally wrote the code last night to reverse the order and put the items back into our inventory system.